Mower is one of 10 Republicans who have filed to run in the primary, which takes place Sept. 13. The other leading challengers are Gail Huff Brown, a former television reporter and wife of one-term Massachusetts Sen. Scott Brown, as well as Karoline Leavitt, who worked as an assistant White House press secretary under Trump and supports his Big Lie.
The NRCC booked almost $3.5 million for advertising in the Boston market, which could be used to put pressure on Pappas as well as Democratic Kuster.